The production of noxious nitrogen oxides (NOx) by internal combustion engines which pollute the atmosphere are undesirable and in many cases are controlled by regulations established by governmental entities. Furthermore, spark ignited engines exhibit abnormal combustion phenomena called “knock”, which occurs when combustion reactions in the unburned zone initiate rapid uncontrolled combustion prior to the arrival of the propagating flame front of a homogenous combustion process. One method for controlling knock includes increasing the flame propagation rate by, for example, improving the mixture homogeneity or by increasing the turbulence level induced by organized charge motion.
One technique for limiting or controlling the combustion temperature of the engine and thus reduce NOx emissions has been to recirculate a portion of the exhaust gas back to the engine air intake to lower the oxygen content in the intake air. This reduces the combustion temperature of the intake charge and in turn reduces the amount of NOx formation during combustion due to lower flame temperatures. In order to recirculate exhaust gas, an exhaust gas recirculation (EGR) line that connects the exhaust manifold to the intake air supply line is provided.
A technique to increase the flame propagation rate to address knock is to have one or more cylinders dedicated to providing EGR flow to the engine intake. When the EGR line is connected with one or more dedicated cylinders, the engine acts as a positive displacement pump to drive the EGR flow, eliminating pumping losses in transporting exhaust to the intake system and allowing a wide range of engine out nitrous oxide emissions to be achieved. Also, since the exhaust from the dedicated cylinder does not escape the engine, it is possible to have alternative combustion processes with the dedicated cylinder(s). In addition, a variable geometry turbocharger is not required to drive EGR flow, facilitating meeting of target air-fuel ratios.
Engines operating with one or more cylinders as dedicated EGR cylinders enjoy greatly simplified controls and pressure management, fewer hardware devices, and other benefits. However, while there is some ability to control the combustion processes such as by running the dedicated EGR cylinder(s) to generate favorable species like hydrogen, the ability to do so is limited since the same fuel is used in the dedicated and non-dedicated cylinders. For example, certain fuels provide high energy density but do not readily produce favorable species such as hydrogen and carbon monoxide, which increase combustion speed, reduce engine knock, and improve fuel economy. Other fuels more readily produced favorable species, but sacrifice energy density and performance.
Thus, there remains a need for additional improvements in systems and methods that include one or more dedicated cylinders dedicated to providing EGR flow to optimize operation, performance, and fuel economy of internal combustion engines.